Atsushi Okamoto

This study documents the first occurrence of pyrochlore in Mongolian carbonatites and provides a case study of the magmatic–hydrothermal evolution of the Lugiin Gol alkaline–carbonatite complex and its Nb–REE mineralization, offering new insights into Early Triassic magmatism and tectonic processes in southern Mongolia. Pyrochlore is the principal Nb‑hosting mineral in the Lugiin Gol carbonatite, whereas REE‑fluorocarbonates (parisite, synchysite, bastnäsite) represent the dominant REE-bearing phases. Petrographic and mineral-chemical data indicate a coupled magmatic–hydrothermal evolution of Nb–REE mineralization: (i) pyrochlore crystallized as inclusions in calcite and apatite from the primary carbonatite melt, establishing the primary Nb budget with limited redistribution during subsequent hydrothermal overprints; (ii) magmatic processes may have partially incorporated REEs into magmatic minerals, particularly calcite, which were subsequently mobilized during hydrothermal activity, redistributed along mineral cleavages to form subordinate REE‑fluorocarbonates, and concentrated within pre-existing structural discontinuities, producing the main REE‑fluorocarbonates and significant REE enrichment. New in situ zircon U–Pb geochronology of coexisting alkaline–silicate rocks yields a weighted mean age of 246 ± 4 Ma, constraining the timing of magma emplacement, with carbonatite emplacement inferred to have been broadly contemporaneous during the Early Triassic. The complex likely formed in a localized extensional setting, with magmatism potentially associated with the closure of the Paleo-Asian Ocean, which is generally considered to mark the final stage of amalgamation of the southern Central Asian Orogenic Belt. The study underscores both the tectonic framework and the mineralization processes that govern the distribution and concentration of critical metals in orogenic carbonatite systems.

In the linear plasma experimental device NUMBER, the densities of ions and atoms in helium plasma were estimated using passive spectroscopic measurements. The collisional-radiative models were employed to derive the densities of atoms and ions from the measured line intensities of each species. The estimated densities of helium species showed that (1) the atom density is one order of magnitude smaller than that estimated from gas pressure before discharge, (2) the density of singly charged ions is one order of magnitude smaller than that of atoms, (3) the doubly charged ions have a density 4 orders of magnitude smaller than the singly charged ions.

The ion temperature in an ECR plasma is determined from the ratio of wavelength-integrated spectra obtained using a spectrometer with sufficient time resolution. The ion temperature during the intermittent negative spikes in floating potential is extracted using the conditional averaging method. For the first time, we have successfully observed a decrease in ion temperature during intermittent events.

A laser Thomson scattering system is developed to obtain anisotropic electron temperature in a pulse operated electron cyclotron resonance plasma device. Injecting a laser with an oblique angle to the external magnetic field and detecting scattering photon from a line of sight along another oblique angle enable us to obtain parallel and perpendicular components of electron temperature. Backward (165°) scattering spectrum corresponds to quasi perpendicular velocity distribution, while that for forward (15°) scattering; quasi parallel. Collecting lenses are set in vacuum to maximize solid angle in a limited space of port, where the laser path and collecting optics share an ICF152 flange. Rayleigh scattering intensity is measured as a function of argon gas pressure. An initial result on residual stray light intensity is equivalent to the argon Rayleigh scattering intensity under ≤ 1 kPa of filled pressure. In order to obtain Thomson scattering spectra, a notch filter type stray light rejection optics, which utilize a reflective type volume holographic grating, is evaluated. Notch width (0.2 nm) and flat transmittance (≥ 90%) outside the notch measured for a prototype show that it is an effective optics.

In the converging field configuration of cylindrical plasma following an ECR plasma source, intermittent variation of floating potential, which is a negative spike with about a ten microsecond duration, is observed. The event is ubiquitous; independent of measurement position and magnetic field. Cross-correlation analysis for two floating potentials at separate positions reveals that a strong correlation exists within the event timescale. The correlation along the magnetic field line suggests an axially elongated structure. The radial extent of the structure is clarified by the radial correlation profile. Different FWHM of the radial profile at the plasma production region and the downstream converging field region is essentially explained by field line tracing.

The ion temperature of the smallest tokamak, the major radius of 0.1 m, is measured using Doppler broadening spectroscopy. Experiments are performed for helium discharge. Ion temperature T_i = 0.7eV is obtained from the Doppler broadened line spectrum of the helium ion. The electron temperature and density measured using line emission intensities of the helium atom are T_e = 4.7eV and n_e = 3.2 × 10¹⁸m⁻³. The major radius R₀ = 0.11m and the minor radius a = 0.03m are obtained from magnetic measurements. Then, the energy flow from the electron to the ion is evaluated as well as ohmic heating and power losses due to atomic processes. The main loss channel for electron stored energy is conduction even though the tokamak is immersed in the residual neutral gas. Total energy confinement time τ_E = 2.3µs is determined from the power balance, which is comparable with that deduced from the neo-Alcator scaling law.

The rise of electron temperature along the magnetic field line was clearly observed in divertor relevant recombining plasma, even though there was no additional electron heating source. Electron temperature obtained at recombination front was approximately 0.5 eV, whereas it increased up to greater than 1 eV at downstream of recombination front. Although this temperature rise is likely common in detached or recombining plasmas, the mechanism has not been understood yet. This report provides a reasonable interpretation of temperature rise along the magnetic field line, for the first time: depletion of low energy electrons due to volumetric recombination leads to deformation of electron energy distribution, resulting in an apparent increase in electron temperature. Our experiment supports this interpretation in that the experimentally observed electron temperature showed good agreement with calculated effective electron temperature.